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Transmission Tweak Promises Big Cost Benefits for Offshore Wind

Offshore wind development is being pushed further out into deeper waters, emphasizing longer, higher-capacity transmission systems. Most newer offshore wind farms from Europe to the U.S. are looking at hundreds of kilometers of transmission lines: the U.K. Crown Estate's Round 3 allocations, interconnection systems from Germany's North Sea to the U.K.'s National Grid Western Link, and the proposed Atlantic Wind Connector in the U.S. Mid-Atlantic.

Those distances are one of a number of factors driving a switch from tried-and-true high-voltage AC (HVAC) to high-voltage DC (HVDC) for proposed offshore grids and as a suitable interface with different grids, including meshing with AC back onshore. (Also in HVDC's favor: superior voltage quality, fewer transmission losses, and better power flow control.)

There's some debate whether HVAC technology can be improved and be viable in performance and costs even when stretched out to 60-100 km distances, but beyond 100 km or so where offshore wind farms are going "there is no question DC is the only option," explains Joel Whitman, principal at Whitman Consulting Group, who led a transmission-themed panel at an offshore power conference last week in Boston. Put some more numbers in HVDC's column: Navigant Consulting expert Kris Torvik projects 30 GW of offshore wind will be connected by HVDC by 2020, which will help to triple the market for high-voltage submarine power cables.

There are some significant hurdles to moving the offshore wind industry from incumbent HVAC to HVDC. Roughly 9,000 km of offshore cable will be installed over the next 15 years, noted Pierre Bernard, formerly with European grid operator Elia Group, speaking on the offshore wind event panel. (The U.K.'s Crown Estate has arrived at roughly the same estimate.) One does not simply order and install new high-voltage offshore cabling, however: very few ships can handle the sheer amounts necessary for increasingly large offshore wind farms, few ports can accommodate them, and an experienced crew is scarce. As a result there's a massive backlog for the few firms who have the resources; Torvik points to a $1-$2 billion backlog in HV cable orders at Prysmian, ABB, and Nexans. Whitman estimates that roughly 80 percent of insurable losses to date in offshore wind have been cable-related, so this one area is arugably offshore wind development's most impactful area to focus on reliability and costs.

Efforts are well in progress to make HVDC less costly in any number of ways. Switching to DC can remove the need for offshore transformers and platforms, and fewer components in the main power system translates to better reliability and efficiency. To that end, GE Power Conversion (formerly known as Converteam, acquired in 2011 and a former unit of Alstom) is touting what it says is a way to dramatically lower costs of offshore wind: swap out the conventional wind turbine transformer with something it calls PassiveBoost. It is essentially a medium-frequency transformer with a diode bridge that converts the 33 kVAC waveform from the turbine power converters into a 50 kVDC one, thus increasing the site's output. Full-scale demonstration trials in the U.K. proved the system's predicted efficiency and footprint (the same size, footprint and weight as the existing turbine generator), according to GE.

Making a direct connection to a HVDC power grid reduces cable cost and eliminates the need for an expensive and complex DC breaker, according to the company. Which leads to the company's bigger claims about the technology: it can lower levelized cost of energy for offshore wind by 11-15 percent. As an apparent thumbs-up from the industry, GE claims Seoniad Vass, director of renewable energy and low-carbon technologies at Scottish Enterprise, was said to be "involved in the early stages of the project."

Lead image: Blue fantasy lightning, via Shutterstock

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